With the increased speed and loading of land transportation machinery such as railroad cars and automobiles, as well as motor bicycles and the like, disc brakes have come to be widely used as braking devices for such vehicles.
Following is a description of an example of a disc brake used for a railroad car.
A disc braking system is a device which produces a braking force by means of friction between a brake disc and a brake lining, and which operates by pressing the brake lining on a frictional surface of the brake disc mounted onto an axle or a wheel so as to produce a braking force and thereby control the speed of a vehicle by reducing the revolution of the axle or the wheel.
When it works, the temperature of a contact surface of the brake lining and the brake disc rises due to a frictional heat, and the temperature of the contact surface tends to increase, because the braking force increases as the speed increases or as the weight of the vehicle increases.
Consequently, there is a need to make a contact between the brake lining and the brake disc as uniform as possible during braking, and to reduce the frictional heat, so as to increase the durability of the brake disc.
FIG. 3 shows a brake lining and a brake disc of a conventional disc brake for a railroad car. FIG. 3 (a) is a planar view of a brake lining as seen from the side of a friction body, and FIG. 3 (b) is a sectional view along line B-B in FIG. 3 (a).
A brake lining 1 is mounted to a brake caliper (not pictured), and has a component such that a friction body 1a, which makes contact with a frictional surface 2a of a brake disc 2, is mounted to a back plate 1b by means of a rivet (not pictured).
A brake caliper is a device which presses the brake lining into contact with the brake disk to produce a braking force by a hydraulic or an air pressure. A pressing force by the brake caliper onto the brake lining is not distributed uniformly on the entire brake lining, but rather, it concentrates on an area where the pressing force is applied, due to the mounting structure between brake lining and brake caliper.
Accordingly, the conventional disc brake has a structure such that the pressing force applied by the brake caliper concentrates only a small area of the brake lining. As a consequence, a significant difference arises between areas of high and low contact pressure of the brake lining and the brake disc, and the contact pressure between the brake lining and the brake disc tends to increase at the area where the pressing force from the brake caliper operates.
In particular, in the case of a high-speed railroad car such as a bullet train, temperature of the brake lining and the brake disc during braking is probably very high, because of increased thermal energy due to friction in areas of high contact pressure. When the temperature rises due to larger contact pressure, the amount of wear increases between the brake lining and the brake disc, and this can cause cracks in the brake disc. It is therefore important to efficiently distribute the thermal energy resulting from friction between these two parts during braking in order to ensure durability of the brake linings and brake discs.
In response to this problem, in recent years, structures have been disclosed for brake linings which aim to achieving uniform contact (uniform contact pressure) between the brake lining and the brake disc (for example, Patent References 1-3).
The brake linings disclosed in Patent References 1-3 have friction bodies divided into a plurality of members and each of the respective friction bodies employ springs or spherical-seated bearings so that they are able to rotate. As a result of such a structure, it becomes possible to achieve a uniform contact pressure between the brake lining and the brake disc.
However, since the each friction bodies are smaller in size, the friction bodies tend to rotate at their attached places due to friction against the brake disc. As a result, members which fasten the friction bodies to a back plate loosen, and there is a possibility that the friction bodies will ultimately fall off.
Therefore, the brake linings disclosed in Patent References 2 and 3 are provided with detent mechanisms. However, the detent mechanisms disclosed in Patent References 2 and 3 require new components other than the friction bodies, springs or spherical-seated bearings, back plates, and fastening components such as rivets, which are needed for the brake linings to achieve uniform contact pressure. Consequently, this increases the likelihood of greater weight and reduced productivity. Moreover, in the structures disclosed in Patent References 2 and 3, there is a possibility that durability of the components cannot be sufficiently ensured, because a torque load is applied to the components used for the detent mechanisms.